Centrally heated or cooled buildings require that some portion of fresh "makeup" air be added continuously to the total volume of circulated air to keep the space fresh, comfortable and healthy. Hence, a corresponding portion of the air which has already been heated or cooled must be exhausted, resulting in a loss of heat energy and a corresponding reduction in the heating or cooling efficiency of the system conditioning the air in the spaces. Heat exchangers are commonly used in the exhaust air and makeup airflow paths of these systems to recover some of the energy from the exhaust air and to induce warmer make-up air during heating processes and cooler makeup air during cooling processes.
So-called "total heat exchangers," which perform moisture exchange as well as heat exchange, serve to introduce warmer makeup air having a higher moisture content during heating process and cooler air having a lower moisture content during cooling process.
Materials used for heat exchangers commonly include metal foils and sheets, plastic films, paper sheets, and the like. Good heat exchange is generally possible with these materials, but moisture exchange cannot easily be performed. Dessicants, or moisture adsorbing materials, are occasionally employed to transfer moisture. With this method, the dessicant merely holds the moisture. To effect transfer moisture between gas streams, the dessicant must be relocated from the gas stream of higher moisture content to the gas stream of lower moisture content, requiring an additional input of mechanical energy. With many dessicant materials, satisfactory performance can be achieved only with the input of additional thermal energy to induce the dessicant to desorb the accumulated moisture.
Heat and moisture exchange are both possible with an exchange-film made of paper. However, water absorbed by the paper from condensation, rain, or moisture present in the air can lead to corrosion, deformation, and mildew growth, and, hence, deterioration of the paper exchange film.
The various types of heat and moisture exchangers in common usage are generally contained within an opaque metal housing and located at or near the building air-handling units in the mechanical room, basement, or rooftop of the building. The nature of moisture exchange requires a very large surface area in contact with the gas stream, and, consequently, total heat exchangers are often very large in size when compared to heat-only exchangers. A larger exchanger in the conventional location requires additional mechanical room space and/or additional load-bearing capacity of the roof in the case of a roof-top unit.
Porous polymeric or ceramic films are capable of transferring both heat and moisture when interposed between air streams of differing energy and moisture states. A system for heat and moisture exchange employing a porous membrane is described in Japanese Laid-Open Patent Application No. 54-145048. A study of heat and moisture transfer through a porous membrane is given in Asaeda, M., L. D. Du, and K. Ikeda. "Experimental Studies of Dehumidification of Air by an Improved Ceramic Membrane," Journal of Chemical Engineering of Japan, 1986, Vol. 19, No. 3. A disadvantage of such porous composite film is that it also permits the exchange of substantial amounts of air between the gas streams, as well as particles, cigarette smoke, cooking odors, harmful fumes, and the like. From the point of view of building indoor air quality, this is undesirable. In order to prevent this contamination of make-up air, the pore volume of a porous film is preferably no more than about 15%, which is difficult and expensive to achieve uniformly. Furthermore, a porous film made to a thickness of 5 to 40 micrometers in order to improve heat exchange efficiency tears easily and is difficult to handle.
Accordingly, a need exists for an efficient heat and moisture exchange apparatus which will not contaminate make-up air.
Additionally, a need exists for heat- and moisture-exchange for use in a system for conditioning air which has excellent heat exchange efficiency, high moisture exchange capability, and serves as a barrier to air flow between exhaust and makeup air streams.